Phosphate Diffusion Coefficients in Soil as Affected by Bulk Density and Water Content

Abstract Phosphate diffusion coefficients (D e ) were determined by the quantity of P that diffused from a soil block with P addition into a soil block without P addition. To compare the results with theoretical concepts and to quantify the influencing factors, D e was also calculated using the equation of Nye (1968). This equation takes into account the P diffusion coefficient in water, D 1 , the volumetric water content, θ, the impedance factor, f, and the buffer power, b, of the soil. The results show that D e strongly depends on volumetric water content whereas the effect of bulk density on D e values was relatively small. If the weighted average buffer power was used, calculated D e values were in good agreement with measured values at higher soil moisture contents. At θ < 0.22 g cm −3 the measured values for D e were smaller than the calculated. This effect is attributed to incomplete contact between the two soil blocks. The only small influence of bulk density on D e is caused by the fact that bulk density affects both θ and b in a way which compensates each other.